We have continued to examine the role of osmotic swelling of secretory granules during exocytosis. This year, we are focusing on exactly how this swelling is accomplished. We examine sea urchin egg cortical granule exocytosis with differential interference contrast light microscopy, phase contrast microscopy, fluorescence microscopy, and intracellular recording to measure cell capacitance and potential. Compromising the integrity of the secretory granule membrane to the extent of allowing free passage of small molecules does not alter calcium stimulated secretion in vitro. Exocytosis proceeds without ions. Granules remain intact in the presence of concentrations of digitonin sufficient to cause lucifer yellow entry. Higher concentrations of digitonin cause the same phase-transition of contents seen with calcium. This phase transition can be inhibited reversibly with osmotic stress. Exocytosis is prevented by including a variety of polymers of different chemical composition and molecular weight in the sea water surrounding the eggs. The increase in membrane capacitance which occurs during exocytosis is not greatly altered by inhibitory concentrations of polymer. Calcium and magnesium reduce the osmolality required to prevent exocytosis. These results suggest that calcium causes swelling by an alteration in the state of the internal granule phase to increase the affinity of this phase for water rather than by inducing ionic fluxes into the granule.